Electrical resistance unit



June 8, 1937.

G. E. MEGOW ELECTRICAL RESISTANCE UNIT Original Filed Sept. 16, 1951 55 02" p 5 MP 0% Patented June 8, 1937 ELECTRICAL RESISTANCE UNIT George E. Megow, South Milwaukee, Wis., aa-

aignor, by mesne assignments, to Globe-Union Inc., Milwaukee, Win, a corporation of Delaware Original application September 18, 1931, Serial Divided and this application February 10, 1934, Serial No. 710,664

7 Claims. (Cl. 201-48) This invention relates to electrical resistance units, more particularly capacity and inductance-free high resistances, and is a division of my application filed September 16th, 1931, Serial 5 No. 563,135 which has matured into Patent No.

Resistance units of the character described are used principally in radio apparatus which necessitates that the units be of unvarying accuracy 10 and stability as to their resistance values irrespective of the surrounding conditions.

The present highly competitive nature of this industry demands that every possible reduction be made in the cost of the units, and the excep- 15 tionally wide range of resistance values required,

which runs from below one thousand ohms upwardly to over ten million ohms together with a demand for reduced size and increased mechanical strength, has made the production of units 90 having all the necessary qualifications extremely To insure absolute stability of the unit, it is essential that the resistance material be protected against contact with moisture. The type of unit best adapted to these needs is a ceramic unit of rod shape with the resistance material forming a lengthwise core therein, but difliculty has been experienced with this type of unit in that the core of resistance material shrinks away 3 from the insulation material surrounding it.

This shrinking results in an undesirable clearance between the core and the insulation which invariably results in breakage of the core. Obviously such breakage of the core changes the 35 resistance characteristics oi. the unit and in some instances results in a dead or open circuit. Another objection to even a slight clearance between the resistance material forming the core and the insulating body is that heat conduction 40 from the core through the insulating body is poor.

With these and other objectionable features of the present ceramic type of unit in mind, this invention has as one of its objects to provide a 45 novel unit of this character wherein a perfect bond is obtained between the resistance core and the insulating material surrounding it.

With the above and other objects in view which vention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiwill appear as the description proceeds, the in-' ment of the hereindisclosed invention may be made as come within the scope of the claims.

In the accompanying drawing, is illustrated several complete examples of the physical embodiment of my invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a longitudinal section view through a completed resistance unit constructed in accordance with this invention;

Figure 2 is a view similar to Figure 1, but illustrating the unit in its condition immediately after extrusion;

Figure 3 is a cross sectional view taken through Figure 2 on the plane of the line 33;

Figure 4 is a side elevation of a unit constructed in accordance with this invention, but having a helically shaped core;

Figure 5 is a cross sectional view taken through Figure 4 on the plane oi. the line 5-5; and

Figures 6 and 7 are cross sectional views through resistance units having cores of diflerently shaped cross section. g

The completed unit constructed in accordance with this invention is illustrated in Figure 1 and comprises a body i of insulating material having a core 2 of resistance material extending longitudinally therethrough. Both the body I and the resistance core 2 are formed of porcelain forming clay or other suitable ceramic material and the resistance core is given the desired degree of conductivity by the addition of carbon black.

At the ends 3 of the unit, the insulating material contains carbon black which is deposited into its pores, in the manner brought out in a copending application of Laurence E. Power, Serial No. 551,608, flied July 18, 1931, to provide low resistance terminals and facilitate the connection of the unit in an electric circuit.

As illustrated in Figure l, and as more at length defined in the said copending application, the conducting material at the ends 3, contacts with the extremities of the resistance core, and metal caps l are preferably pressed onto the ends 3 to enable wire leads, not shown, to be soldered or otherwise electrically connected thereto so that the connection of the unit in an electric circuit is facilitated. 7

As defined at length in my copending application, Serial No. 563,135, of which this is a division, v

the manufacture of the unit as illustrated in Figure 1, comprises generally the preparation of two batches of porcelain forming clay, one contalning a predetermined percentage of carbon black and the other without; the simultaneous extrusion of both materials while still moist into long rods having the material containing carbon black forming a resistance core within the nonconducting material, and the cutting of the rods to appropriate lengths; After the rods are cut into pieces of the desired length the material is still fairly moist, but the pieces will maintain their shape. They are then dried at approximately room temperature (68 degrees F.) for three or four hours and then baked at a temperature sufficient to increase their porosity. At the completion of this baking period, the ends of the units are immersed in a carbonaceous liql5 uid. which may be a phenol condensation product varnish dissolved in denatured alcohol. This liquid contains a relatively high percentage of fixed carbon which penetrates into the pores of the end portions of the unit.

After having been immersed, the pieces are subiected to a temperature at which the carbon is freed from its carrier and becomes fixed in the porous ends of the structure. The unit is also vitrified during this latter baking to a substantially glass-like hardness. At this stage, the

metal caps 4 may be pressed onto the ends of the units.

Several methods have been developed for insur ing the desired intimacy of contact between the core and its insulating body. In each instance a difference in shrinkage between the resistance core and the insulation material surrounding it is obtained, with the insulating material shrinking more in volume than the resistance core so that during the baking processes the core is securely gripped by the insulating shell surrounding it.

One method of obtaining the desired difference in shrinkage of the materials consists in dividing the porcelain forming clay which has the desired 4. amount of carbon black mixed with it and of which the resistance core is formed. into two batches of seventy per cent (70%) and thirty per cent (30%) by weight. The larger portion is calcined at approximately twenty-two hundred 5 (2200) degrees 1"., a point on the low range of vitrification which reduces the shrinkage of the material, and then ground and mixed with the uncalcined material which is the batch of thirty per cent (30%) by weight.

The porcelain forming clay of which the insulating body is formed is not calcined, and hence after the extrusion the greater shrinkage of the outer insulating material during baking binds the resistance core and thus insures an intimate contact between the two materials.

All of the material of which the resistance core is formed is not calcined, so that the uncalcined portion provides a moist carrier for the dry calclined portion.

00 A difference in shrinkage may be also obtained by selecting a material for the resistance core which vitrifies at a higher temperature than the material of the outer insulation body. To this end the material for the insulating body contains approximately two per cent (2%) by weight of sodium .or potassium oxides whereas the resistance core is formed of porcelain forming clay material, consists in reducing the material for the insulating body to a finer degree of sub-division than the material for the resistance core. Both materials are a porcelain forming clay with the resistance material containing a percentage of carbon black, and it has been found that this material in different degrees of subdivision has different rates of shrinkage. Hence. during the firing, the insulating body being in a finer state of sub-division, shrinks to a greater degree.

Intimacy of contact between the core and the insulating material surrounding it also results from the specific construction of the extruding die. The shape of the extruding die is such that the materials forced therefrom are compressed to a substantial degree so that any voids or clearance between the core and the insulating shell is impossible.

In some instances it has been found desirable to have the resistance material in a moist or semi-fluid state during the extrusion. Compression of the materials at the point of extrusion is thus impossible. In this case the extruded rods are allowed to dry for a predetermined period of time and with the insulating material still in a plastic state, the rods are compressed ina compression die or any other suitable mechanism.

The shape of the core 2 may be varied and in Figures 4 and 5 a unit having a helically shaped resistance core is illustrated, thus providing a core of greater overall length than that of the unit. If greater contact area is desired between the core and the surrounding insulation material, the cores may be of irregular shape in crosssection, as illustrated in Figures 6 and '7.

From the foregoing description taken in connection with the accompanying drawing, it will be readily apparent to those skilled in the art to which an invention of the character described appertains, that a ceramic type unit is provided in which a core of resistance material is embedded in a body of insulating material in a practical and economical manner, and in which a perfect bond is secured between the core per se andits enclosing envelope.

What I claim as my invention is:

1. An electrical resistor element comprising a rod of ceramic insulating material, and a core of the same material but containing a predetermined percentage of conducting material, embedded in the rod and maintained in a state of compression by the outer insulating material.

2. An electrical resistor element comprising a body of porcelain forming clay having a core of porcelain forming clay mixed with a conducting material and in which the core is held in a state of compression by the outer body of porcelain forming clay.

3. An electrical resistor element comprising a core of porcelain forming clay mixed with a conducting material encased in a shell of porcelain forming clay having a greater degree of shrinkage upon subiection to heat than the porcelain forming clay mixture of which the core is composed.

4. An electrical resistor element comprisinga rod-like body having a core of porcelain forming clay containing a percentage of carbon black encased within and compressed by a shell of porcelain forming clay having a greater degree of shrinkage upon subjection to heat than the material forming the core, and said body being vitrified throughout its entire structure.

5. An electrical resistor element comprising a core of vitrifiable material having a conducting material mixed therewith, and a shell of vitrifiable insulating material encasing said core, said shell being in a permanent state of contraction so that the core is maintained in a permanent state of compression and the entire element is permanently a homogeneous mass regardless of temperature changes to which the element may be subjected during use.

6. An electrical resistor element comprising a thread-like core composed of a vitrifiable material containing a percentage of conducting material, encased in a shell of vitrifiable insulating material whose cross sectional area is considerably greai er than that of the cross section of l! the thread-like core, and said shell being in a permanent state of contraction and maintaining the thread-like core in a permanent state of compression so as to insure perfect bond between the core and shell and protect the thread-like core against breakage.

7. An electrical resistor element comprising a core of vitrifiable material having a conducting material mixed therewith, and a shell of the same vitrifiable insulating material encasing said core, the shell being composed of initially relatively finer particles than the core material to produce a shrinkage factor in excess of that of the core, said resistor being vitrified throughout.

GEORGE E. MEGOW. 

